Gearboxes are used in many applications to provide speed and torque conversions from a rotating power source to another device. In some applications, such as a cooling tower fan, the gearbox may be arranged such that its output shaft extends in the downward direction towards the ground. This configuration, oftentimes referred to as an output shaft down configuration, presents challenges in retaining lubricant inside the gearbox while rotating the output shaft.
The output shaft typically is supported by one or more bearings requiring lubrication. A splash lubrication scheme, involving a liquid lubricant such as oil, is commonly used for this purpose. In such a lubrication scheme, the oil collects near the bottom of the gearbox and around the opening through which the output shaft extends. As a result, the oil can leak through the opening especially when the various sealing mechanisms begin to wear.
To address this issue, some gearboxes employ different lubrication schemes for the output shaft bearings and the other gearbox bearings. For instance, a non-splash, grease lubrication scheme may be used for the output shaft bearings, whereas a splash, oil lubrication scheme may be used for the other gearbox bearings. However, the use of two separate lubrication schemes increases the cost and complexity of the gearbox, and makes maintenance activities more cumbersome.
Another treatment for reducing leakage is to use a radial lip seal where the output shaft exits the gearbox. The radial lip seal may enable lubrication of the output shaft bearings with a splash lubrication scheme. However, the radial lip seal must directly contact the rotating output shaft. As a result, the slightest defect or wear at this interface may compromise the ability of the radial lip seal to prevent leakage, which in turn can lead to catastrophic failure. Also, friction between the radial lip seal and the output shaft may inhibit rotation of the output shaft and consequently result in power losses.
Another method of preventing leakage involves creating a dry well around the output shaft. FIG. 1 illustrates one known dry well arrangement 100. Here, the output shaft 120 is made of a hollow outer portion 122 and a solid inner portion 124, with an axial end of the hollow outer portion 122 being secured to an axial end of the solid inner portion 124 at a weld 125. The hollow outer portion 122 is supported by a bearing 130. The solid inner portion 124 extends through the hollow outer portion 122 and exits the gearbox through opening 140. A tube 150 is fixed in the opening 140 and extends into the gearbox between the hollow outer portion 122 and the solid inner portion 124. The upper end of the tube 150 is configured so that it rises above the surface of a lubricant bath 160 inside the gearbox. Accordingly, lubricant generally cannot escape the gearbox through the hollow interior of the tube 150. The tube 150 thus creates a dry well for the solid inner shaft 124, while the outer portion 122 is seated against and is guided by the bearing 130.
One downside to the conventional dry well arrangement is that the output shaft 120 must be constructed in two separate pieces (i.e., a hollow outer portion 122 and a solid inner portion 124), subsequently welded or otherwise fixed together. This two-piece construction can reduce the strength of the output shaft and increase the complexity of its assembly (e.g., welding typically is required). Furthermore, the conventional dry well arrangement lacks any sealing redundancy at the interface between the tube and the inner wall of the output shaft opening.